382 THE FOOD OF PLANTS 



with organic food, although they arc able to assimilate carbon dioxide 

 (Sect. 63), and presumably but few organic substances will enable chloro- 

 phyllous plants to continue their life and growth when the assimilation 

 of carbon dioxide is inhibited. 



The characteristics of an organism are by no means invariable con- 

 stants, and it has already been mentioned (Sects. 64, 65) that the nutritive 

 requirements may alter as development progresses. Spores of Asper- 

 gilhts nigcr germinate but slowly on glycerine, acetic acid, or alcohol 1 , 

 whereas these substances afford fairly good nutrient material for the 

 adult fungus. Apparently in many plants a supply of proteid material 

 including peptones is of importance or absolutely essential for embryonic 

 development. 



The following are additional examples : Bacillus perlibratus will not grow upon 

 tartaric acid, but will upon acetic acid 2 , which for most mould-fungi has the lesser 

 nutritive value. For fungi and many bacteria dextro-rotatory tartaric acid is a better 

 food-material than laevo-rotatory, which is, however, preferred by certain bacteria, 

 whereas other fungi and bacteria again grow equally well upon both of these stereo- 

 isomeric compounds (Pfeffer, 1. c., p. 220). Fermenting organisms behave similarly ; 

 many must have a supply of sugar, whereas others primarily require tartaric or 

 formic acids, or other substances. Both good and bad nutritive substances are 

 found among benzole and methane derivatives, and certain fungi can grow as well 

 upon quinic acid as upon sugar, whereas the nutritive values of stereoisomeric 

 compounds such as tartaric, fumaric and malic acids 3 may be entirely different. 

 Although good nutritive substances are most abundant among the groups of carbo- 

 hydrates and proteids, many organisms are unable to grow upon a mixture of 

 peptone and sugar, which is the most suitable medium for most fungi. 



The nutritive value and physiological importance of a substance are 

 dependent upon properties to which no attention is paid in chemical 

 classification and of which the constitutional formula affords no indication. 

 The characters of the given plant are of equal importance in determining 

 the result produced, and it is impossible to tell from the result what 

 are the complicated and interacting factors which lead to it. There is 

 no doubt, however, that chemical affinities largely determine the fate of 

 a given substance and whether it shall be used in metabolism or left 

 intact. Just as a nut may be cracked by a blow which is applied at the 

 proper point but is ineffective elsewhere, so also may a substance be 

 decomposed by the protoplast only when the interactions resulting from 



1 Duclaux, Ann. d. 1'Inst. Pasteur, 1889, T. in, p. 112. This is often the case in fungi. Cf. 

 also Bourquelot u. Graziani, Bot. Centralbl., 1893, Bd. LV, p. 326. 

 " Beyerinck, Centralbl. f. Bact. , 1893, Bd. xiv, p. 834. 

 1 Wehmer, Beitrage z. Kenntniss einheim. Pilze, 1895, n, p. 98. Cf. also Pfeffer, 1. c., p. 228. 



